Ever since the discovery of the caspases almost 20 years ago there have been reports of non-apoptotic roles for apoptotic caspases. Perhaps the clearest paradigm is caspase-8 (casp8), which although essential for triggering the extrinsic apoptotic pathway, is also required for the survival of certain cell populations during development. For example, deletion studies in mice reveal that casp8, its adapter FADD, and its regulator FLIPL may drive either clonal expansion/proliferation or apoptosis of lymphocytes, cardiomyocytes, and some epithelial cell types. This proposal seeks to provide biochemical evidence for the choice between survival and death, while also having the potential to reveal downstream targets of the survival pathway. Thus we provide proof-of-concept tools and knowledge that can lead to the development of compounds to interfere therapeutically in the survival/death axis of proliferative diseases such as cancer. The overarching hypothesis The survival of cells entering the proliferation program that depends on casp8 activity relies criticaly on the conformation of the casp8 enzyme and cleavage of a limited number of downstream targets will be dissected in this study. We propose three interlinked aims to test this hypothesis by defining the biochemical nature of the casp8 enzyme that drives the survival events, defining the targets of this enzyme, and understanding how casp8 promotes survival, and not apoptosis, in specific experimental cellular contexts. Each aim can be accomplished independently, and the outcome of each aim will substantially further knowledge of the mechanisms of cell survival mediated by casp8. However, the full power of this project is realized when the data from Aim 1 (fundamental mechanisms of casp8 specificity) is coupled to Aim 2 (endogenous pro-survival casp8 targets), and the outcomes of Aims 1 and 2 used to inform the appropriate experimental paradigms for defining the cell's decision between survival and apoptosis (Aim 3). This proposal is significant because it seeks to reveal the molecular mechanism used by casp8 to delineate survival from death, and a successful outcome may impact directly on the selection of protein and small molecule pharmacologic tools that are under clinical development as cancer therapeutics.